1. Field of the Invention
The present invention relates to electron beam lithography, and more particularly, to a method of compensating for pattern dimension variation caused by a re-scattering effect of the electron beam occurring when a resist is exposed to the electron beam.
2. Description of the Related Art
Electron beam lithography is a technique used in patterning a material layer formed on a substrate in a desired pattern. This entails the process of coating an electron beam resist on a material layer; writing a desired pattern with an electron beam (referred to in the art as an "exposure"); developing the electron beam resist; and etching the material layer by using the electron beam resist pattern formed using the desired pattern as a mask. Electron beam lithography can be used to form a predetermined material layer pattern directly forming an integrated circuit on the substrate, however, in general, electron beam lithography is used to fabricate a photomask for use in photolithography.
Referring to FIG. 1, the process for fabricating the photomask will be described in greater detail. The process comprises the steps of: coating an electron beam resist 130 on an opaque film 120 (in the case of a phase shift mask, a phase shifting layer is available, hereinafter described simply as an opaque film) formed on a transparent substrate 110; writing a desired pattern with an electron beam 150; developing the electron beam resist 130 by using a difference of solubility depending on writing of the electron beam; and etching the opaque film 120 by using the formed resist pattern as a mask.
However, the electron beam 150 does not only expose the desired portion of the electron beam resist 130, as the electron beam 150 is reflected on the surface of the opaque film 120 or scattered by collisions with atoms of a resist material in the electron beam resist 130 as marked 170 in FIG. 1. Also, the electron beam 150 is reflected in the electron beam resist 130 or on the surface of the electron beam resist 130 and at the lower plane of an objective lens 140 of an electron beam writer and, as a consequence, the electron beam 150 exposes an undesired portion of the electron beam resist 130 as marked 160 in FIG. 1.
A quantity (a dose) by which the electron beam resist 130 is exposed an extra amount by scattering of the electron beam 150 as described above, is shown in FIG. 2. As shown in FIG. 2, the electron beam resist can be additionally exposed from a region in which a pattern is written with the electron beam, that is, from an edge of the pattern to a maximum distance of 10 cm. Close to the edge of the pattern, the dose can be as high as 25% of the original exposure dose. In FIG. 2, an additional exposure 210 affecting from the region in which a pattern is written with the electron beam, to approximately 10 .mu.m, is caused by forward scattering and backward scattering of the electron beam indicated by reference numeral 170 in FIG. 1, and an additional exposure 220 affecting to approximately 10 cm is caused by re-scattering of the electron beam indicated by reference numeral 160 in FIG. 1. In conclusion, these additional exposures deteriorate the accuracy of the opaque film pattern, and cause a critical dimension (CD) error. The pattern dimension variation caused by the former additional exposure 210 is referred to as a proximity effect, and the pattern dimension variation caused by the latter additional exposure 220 is referred to as a re-scattering effect (multiple scattering effect or a fogging effect) of the electron beam.
The re-scattering effect of the electron beam affects a wide range (Considering the integration of a current integrated circuit, 10 cm is a very wide range.), and since a dose caused by the additional exposure 220 is relatively small, the effect has not been ascertained, and no compensation method is well-known. However, the pattern dimension variation of the photomask caused by the re-scattering effect of the electron beam is estimated to be about 10.about.20 nm when an electron beam dose is 8 .mu.C/cm.sup.2 at an accelerating voltage of 10 keV, and the pattern dimension variation of the photomask greatly affects the manufacture of more highly-integrated circuits.
On the other hand, the re-scattering effect of the electron beam is introduced, and a method for forming the lower plane of the objective lens in which the re-scattered electron beam is reflected, of a material with a low atomic number, as a method for reducing this effect is disclosed in, Norio Saitou et al., "Multiple Scattered E-beam Effect in Electron Beam Lithography", SPIE Vol. 1465, pp. 185-p. 191, 1991. That is, it is reported in the paper that an additional dose caused by the re-scattering effect of the electron beam when the lower plane of the objective lens is formed of copper, aluminum, and carbon, respectively, was measured, and the re-scattering effect of the electron beam was lowest when carbon was adopted. However, it is shown in FIG. 2 that the re-scattering effect is not remarkably reduced even if carbon is adopted. In FIG. 2, the chart of symbol ".smallcircle." applies to the case where aluminum is adopted, and the chart of symbol ".quadrature." applies to the case where carbon is adopted.
Also, a method for reducing the re-scattering effect by absorbing the re-scattered electron beam by attaching an absorber plate in which a honeycomb groove is formed at the lower plane of the objective lens, is disclosed in Naoharu Shimomura et al., "Reduction of Fogging Effect caused by Scattered Electron in an Electron Beam System", SPIE Vol. 3748, pp. 125-p. 132, 1999. However, it is also not possible for all re-scattered electrons to be absorbed by this method, and there is a limitation in reducing the re-scattering effect.